1. Field of the Invention
This invention relates generally to the field of longwall mining and more particularly to a method for recovery of chain pillars separating development entries adjacent a longwall or panel to be mined.
2. Description of the Prior Art
Longwall mining in the United States requires that areas to be mined are typically blocked out by a system of parallel entries variously used for the passage of fresh and return air, belt conveyors or electrical cables. With the enactment of various Federal and state mining laws since 1969, the minimum number of adjacent entries that can presently be developed without "special permits" is two, and frequently sets of three or more are employed. Conventionally, the longwall mining face extends from outboard entry to outboard entry of a development panel. As the panel is mined in retreat, considerable coal is lost in the rows of chain pillars that remain standing subsequent to the mining operation. This practice not only wastes valuable resource but can also cause considerable ground pressure-related problems in the tailgate entry of succeeding panels.
In the past, temporary or permanent roof supports such as wooden cribs or posts have been installed in the development entries and crosscuts to allow mining through the entry and its adjacent pillars. However, this has been done only to alleviate adverse mining conditions or to facilitate production in a specific local area of the mine. Furthermore, such supports must either be removed by hand or if left in place they slow down the mining machines. Consequently, such methods have not been used longer than necessary to escape the region in which the particular problem exists. In view of this prior art, the inventor sought to develop a method for removal of chain pillars during and as an integral part of a longwall mining operation without impairing production or creating hazardous conditions.
A known technique adaptable to longwall mining is that of "pump packing" which is described in a paper entitled "Review of British and American Coal Mining Technology", prepared for presentation at the AIME annual meeting, New York, New York, Preprint No. 75 -F-76. Pump packing, as described in this publication, involves mixing broken coal or other minerals with a binder such as bentonite and water to form a thixotropic material. This material flows under pressure and solidifies rapidly when the pressure is relieved. It has been used by the British to construct separate pack walls. As noted in the reference paper, it has also been used in filling an entry intersecting a longwall panel. Its low strength is said to facilitate cutting through the filled entry by a shearer so that the longwall advance is not interrupted. Thus, pump packing is seen to be a convenient technique for providing a "natural" or homogeneous roof support and as such facilitates the practice of method to be described. Sofar as the inventor is aware, however, pump packing has not been used or suggested as an integral part of a continuous mining operation involving chain pillar recovery.
The invention to be described is of particular utility in the underground mining of thick mineral seams, by which is meant herein seams too thick to work effectively as a single conventional lift with available machinery and supports. In accordance with a known method, thick seam mining in Europe involves driving parallel development entries into the bottom of the seam to form a longwall. The solid face is cut with a bidirectional drum shearer which loads an armored face conveyor. This operation proceeds under the protection of a row of adjacent mine roof supports each extending transverse to the solid face. As the shearer passes each position along the face, the roof supports are progressively advanced to support the roof while mineral from overlying strata on the gob side is allowed to cave for subsequent loading on a gob conveyor. The redesign of existing minor roof support structures for use in this type of mining and their integration in a system for such mining is the subject of a separate invention. These features are detailed in commonly assigned application Ser. No. 664,437, entitled MINE ROOF SUPPORT AND METHOD IN THICK SEAM LONGWALL MINING filed concurrently herewith. The chain pillar problems referenced above in conventional U.S. longwalling are of equal if not greater concern in the mining of these thick seams. In such mining the greater share of mineral recovery is derived from caving and loading of broken mineral. Recovery of chain pillars in longwalling a thick seam will be seen to enhance the amount of recovery of caved coal. Also, where such a thick seam is mined in two or more lifts, chain pillar removal is particularly helpful in destressing the roof of lower lifts.